Supplementary Figure S1. Ancestral state reconstruc�on of sidewinding specializa�on using maximum likelihood es�mates for discrete char- acters (Pagel 1994), as implemented by the ace func�on of the R package APE (Paradis et al. 2004). Pie charts at nodes show the likelihood of a specializa�on for sidewinding locomo�on vs. a generalist state. We used the phylogeny from Alencar et al. (2016) and the narrowest catego- riza�on of sidewinding specializa�on, coding listed in Table S1 as sidewinding specialists. If we assume that all nodes with >50% likeli- hood of having the sidewinding state were in fact sidewinders, then specialized sidewinding has evolved 5 �mes, poten�ally with a reversal (Bitis schneideri, which may show popula�on differences in sidewinding tendency, Table S2). Note that one species in a sidewinding clade, Pseudocerastes urarachnoides, was only recently described (Bostanchi et al. 2006) a�er having been previously been mistaken for one of its congeners, and it is therefore uncertain whether it is similarly specialized for sidewinding.

Bitis xeropaga Bitis rubida Bitis cornuta Bitis armata Bitis atropos Bitis caudalis Bitis schneideri Bitis peringueyi Bitis rhinoceros Bitis gabonica Bitis nasicornis Bitis parviocula Bitis worthingtoni Bitis arietans Proatheris superciliaris Causus defilippii Causus rhombeatus Causus resimus Causus lichtensteinii Atheris chlorechis Atheris squamigera Atheris hispida Atheris desaixi Atheris nitschei Atheris ceratophora Atheris matildae Atheris barbouri ? Pseudocerastes urarachnoides Pseudocerastes persicus Pseudocerastes fieldi Eristicophis macmahoni Vipera nikolskii Vipera berus Vipera barani Vipera seoanei Vipera kaznakovi Vipera orlovi Vipera ursinii Vipera eriwanensis Vipera dinniki Vipera lotievi Vipera renardi Vipera anatolica Vipera aspis Vipera latastei Vipera transcaucasiana Vipera ammodytes Daboia mauritanica Daboia deserti Daboia palaestinae Daboia siamensis Macrovipera schweizeri Macrovipera lebetina Montivipera latifii Montivipera raddei Montivipera bornmuelleri Montivipera albizona Montivipera wagneri Montivipera xanthina Cerastes vipera Cerastes cerastes Cerastes gasperettii Echis ocellatus Echis jogeri Echis khosatzkii Echis borkini Echis leucogaster Echis pyramidum Echis coloratus Echis omanensis Echis carinatus malabaricus Trimeresurus trigonocephalus Trimeresurus puniceus Trimeresurus wiroti Trimeresurus hageni Trimeresurus schultzei Trimeresurus mcgregori Trimeresurus flavomaculatus Trimeresurus malcolmi Trimeresurus septentrionalis Trimeresurus andersonii Trimeresurus purpureomaculatus Trimeresurus cantori Trimeresurus fasciatus Trimeresurus insularis Trimeresurus kanburiensis Trimeresurus venustus Trimeresurus macrops Trimeresurus medoensis Trimeresurus yunnanensis Trimeresurus gumprechti Trimeresurus truongsonensis Trimeresurus vogeli Trimeresurus sichuanensis Trimeresurus buniana Trimeresurus barati Trimeresurus sabahi Trimeresurus fucatus Trimeresurus popeiorum Trimeresurus nebularis Trimeresurus tibetanus Lachesis melanocephala Lachesis stenophrys Lachesis muta Lachesis acrochorda Sistrurus catenatus Sistrurus miliarius Crotalus intermedius Crotalus transversus Crotalus tancitarensis Crotalus pricei Crotalus adamanteus Crotalus mitchellii Crotalus tigris Crotalus scutulatus Crotalus cerberus Crotalus oreganus Crotalus viridis Crotalus ruber Crotalus tortugensis Crotalus atrox Crotalus catalinensis Crotalus willardi Crotalus tzcaban Crotalus simus Crotalus durissus Crotalus culminatus Crotalus molossus Crotalus basiliscus Crotalus totonacus Crotalus ornatus Crotalus horridus Crotalus stejnegeri Crotalus lannomi Crotalus ericsmithi Crotalus aquilus Crotalus lepidus Crotalus triseriatus Crotalus pusillus Crotalus ravus Crotalus cerastes Crotalus polystictus Crotalus enyo Agkistrodon piscivorus Agkistrodon taylori Agkistrodon howardgloydi Agkistrodon russeolus Agkistrodon bilineatus Agkistrodon contortrix Bothrocophias microphthalmus Bothrocophias hyoprora Bothrops lojanus Bothrocophias campbelli Bothrops pictus Bothrops ammodytoides Bothrops alternatus Bothrops fonsecai Bothrops cotiara Bothrops itapetiningae Bothrops insularis Bothrops jararaca Bothrops alcatraz Bothrops barnetti Bothrops matogrossensis Bothrops pauloensis Bothrops pubescens Bothrops diporus Bothrops erythromelas Bothrops lutzi Bothrops neuwiedi Bothrops marmoratus Bothrops caribbaeus Bothrops lanceolatus Bothrops asper Bothrops isabelae Bothrops atrox Bothrops colombiensis Bothrops marajoensis Bothrops leucurus Bothrops moojeni Bothrops osbornei Bothrops punctatus Bothrops brazili Bothrops jararacussu Bothrops pulchra Bothrops taeniata Bothrops chloromelas Bothrops bilineata Porthidium dunni Porthidium hespere Porthidium ophryomegas Porthidium nasutum Porthidium porrasi Porthidium lansbergii Porthidium arcosae Porthidium yucatanicum Atropoides picadoi Cerrophidion petlalcalensis Cerrophidion tzotzilorum Cerrophidion godmani Cerrophidion sasai Atropoides mexicanus Atropoides olmec Atropoides nummifer Atropoides occiduus Atropoides indomitus Ophryacus undulatus Mixcoatlus barbouri Mixcoatlus browni Mixcoatlus melanurus Bothriechis schlegelii Bothriechis supracilliaris Bothriechis bicolor Bothriechis marchi Bothriechis thalassinus Bothriechis rowleyi Bothriechis aurifer Bothriechis lateralis Bothriechis guifarroi Bothriechis nigroviridis Gloydius ussuriensis Gloydius brevicaudus Gloydius blomhoffi Gloydius tsushimaensis Gloydius intermedius Gloydius halys Gloydius shedaoensis Gloydius saxatilis Gloydius strauchi Gloydius liupanensis Ovophis okinavensis Protobothrops tokarensis Protobothrops flavoviridis Protobothrops mucrosquamatus Protobothrops elegans Protobothrops maolanensis Protobothrops dabieshanensis Protobothrops xiangchengensis Protobothrops jerdonii Protobothrops cornutus Protobothrops mangshanensis Protobothrops sieversorum Protobothrops kaulbacki Ovophis tonkinensis Ovophis monticola Ovophis zayuensis Calloselasma rhodostoma Hypnale nepa Hypnale hypnale Hypnale zara Tropidolaemus subannulatus Tropidolaemus wagleri Deinagkistrodon acutus Garthius chaseni Azemiops feae